This invention relates to systems for supplying ink to an orifice array in an ink jet head and, more particularly, to a new and improved ink supply system for an ink jet head.
In the copending Hine et al Application Ser. No. 43,369, filed Apr. 28, 1987, now U.S. Pat. No. 4,814,786, hot melt ink supplied to an ink jet head is circulated continuously by thermal convection to maintain pigment in suspension and to transfer ink from the region of the ink jet orifices to a deaerator. Although such thermal circulation is effective, it consumes energy and may raise the ink to temperatures not otherwise required for operation of the ink jet system.
In many ink jet systems, the proper operation of the ink jet is dependent upon the hydrostatic pressure of the ink supplied to the ink jet orifices. In some systems, such as described, for example, in the Sicking et al. U.S. Pat. No. 4,475,116, ink is supplied periodically to a reservoir on the ink jet head from a remote reservoir and, unless complex pressure control arrangements such as the bladder system described in that patent are provided, the change in level of the ink in the ink jet head reservoir between the maximum and minimum ink level conditions may interfere with the operation of the ink jet system.
As described, for example, in the Kasugayama et al. U.S. Pat. No. 4,419,677, insufficient hydrostatic pressure at the orifice of an ink jet head can cause the ink meniscus to retract within the orifice and, to overcome this condition, pressure must be applied. As described in that patent, air pressure is applied to the ink in the reservoir through a vent which normally maintains the reservoir at atmospheric pressure so as to force ink into the orifice, purging air from the ink jet head and restoring the ink meniscus to the proper place in the orifice. In a similar way, bubbles which may accumulate in the ink jet head can be ejected by applying increased pressure to the liquid in the reservoir through the orifice vent. On the other hand, as also described in that patent, excessive hydrostatic pressure at the ink jet orifice can cause the ink to leak from the orifice, producing a similarly undesirable condition.
Furthermore, when one or more air bubbles have formed or debris has accumulated in an ink jet head, interfering with the operation of the system, conventional ink jet systems, such as described in the Kasugayama et al. U.S. Pat. No. 4,419,677 and in the DeYoung U.S. Pat. No. 4,658,274, apply pressure to the ink in the reservoirs so as to eject ink out of the ink jet head through the orifices, thereby carrying the trapped air with it. Such outflow purging systems necessarily require relatively high-capacity ink capture and cleaning devices to collect and remove the substantial quantities of ink which are ejected through the orifices during purging processes.